I. Technical Field
The present invention relates to a hydrodynamic bearing device that supports a shaft member so as to be freely rotatable in a non-contact manner by an action of the dynamic pressure of lubricating oil generated within a radial bearing gap. This hydrodynamic bearing device is suitable for use in a spindle motor for information equipment including magnetic disc devices such as an HDD and an FDD, optical disc devices such as a CD-ROM drive, a CD-R/RW drive, and a DVD-ROM/RAM drive, and magnetooptical disc devices such as an MD drive and an MO drive, a polygon scanner motor of a laser beam printer (LBP), a color wheel for a projector, or a small motor of electric equipment such as an axial fan.
II. Description of the Related Art
For the aforementioned various motors, improvement of rotational accuracy, increase of an operation speed, reduction of a cost, reduction of noises and the like are needed. One of components that determine those performance requirements is a bearing for supporting a spindle of the motor. In recent years, the use of a hydrodynamic bearing device having excellent characteristics related to the above performance requirements is studied, or such a hydrodynamic bearing device has been actually used.
This type of hydrodynamic bearing device includes a radial bearing portion for supporting a shaft member to be freely rotatable in a radial direction and a thrust bearing portion for supporting the shaft member to be freely rotatable in a thrust direction. The radial bearing portion generates oil film in a radial bearing gap that is formed between an inner circumferential surface of a bearing sleeve and an outer circumferential surface of the shaft member, by an action of a dynamic pressure generated by relative rotation between the shaft member and the bearing sleeve, thereby supporting the shaft member in the radial direction in a non-contact manner. As the thrust bearing portion, a type which forms oil film in a bearing gap (thrust bearing gap) by an action of a dynamic pressure so as to support the shaft member in a non-contact manner (this type is called as a hydrodynamic bearing) like the radial bearing portion and a type which supports an end of the shaft member by a thrust plate in a contact manner (this type is called as a pivot bearing) are known.
According to a conventional technique, the shaft member used in this type of hydrodynamic pressure bearing device is fabricated by roughly shaping metal material by a turning process and thereafter grinding the outer circumferential surface of the shaped metal material and the like which require high fabrication accuracy (see Japanese Patent Laid-Open Publication No. 2002-310159).
In a hydrodynamic bearing device that is incorporated in a spindle motor of a disc device such as an HDD, a member for supporting a disc, e.g., a disc hub, is press fitted and fixed at the tip end of the shaft member. When the disc hub is press fitted while being inclined, shaft movement in an axial direction increases. Therefore, after the hydrodynamic bearing device is assembled, it is necessary to correct inclination while the movement of the shaft is measured. This correction of inclination increases the cost of the hydrodynamic bearing device. Moreover, when the disc hub is inclined, the press-fitting force becomes excessively large. Thus, a large load is applied to various portions of the bearing device, so that the decrease of the accuracy, the decrease of strength of a bonded portion, and the like may be caused.